1. Field of the Invention
This invention relates generally to organoborane amine complexes and, more particularly, to polymerizable compositions, especially acrylic adhesives, that incorporate polymerization initiator systems based on the organoborane amine complexes. This invention further relates to methods for bonding substrates, particularly low surface energy substrates, using such compositions.
2. Description of the Related Art
An efficient, effective means for adhesively bonding low surface energy substrates such as polyethylene, polypropylene and polytetrafluoroethylene (e.g., TEFLON) has long been sought. The difficulties in adhesively bonding these materials are well known. See, for example, "Adhesion Problems at Polymer Surfaces" by D. M. Brewis that appeared in Progress in Rubber and Plastic Technology, volume 1, page 1 (1985). The conventional approaches typically function by: (1) increasing the surface energy of the substrate (to more closely match the surface energies of the substrate and the adhesive thereby promoting better wetting of the substrate by the adhesive) and/or (2) eliminating additives and low molecular weight polymer fractions in the substrate that can migrate to the substrate surface and adversely affect adhesion by forming a weak boundary layer.
As a result, the conventional approaches often use complex and costly substrate surface preparation techniques such as flame treatment, corona discharge, plasma treatment, oxidation by ozone or oxidizing acids, and sputter etching. Alternatively, the substrate surface may be primed by coating it with a high surface energy material. However, to achieve adequate adhesion of the primer, it may be necessary to first use the surface preparation techniques described above. All of these techniques are well known, as reported in Treatise on Adhesion and Adhesives (J. D. Minford, editor, Marcel Dekker, 1991, New York, volume 7, pages 333 to 435). The known approaches are frequently customized for use with specific substrates. As a result, they may not be useful for bonding low surface energy substrates generally.
Moreover, the complexity and cost of the presently known approaches do not render them particularly suitable for use by the retail consumer (e.g., home repairs, do-it-yourselfers, etc.) or in low volume operations. One vexing problem is the repair of many inexpensive everyday household articles that are made of polyethylene, polypropylene or polystyrene such as trash baskets, laundry baskets and toys.
Consequently, there has been a considerable and long felt need for a simple, easy to use adhesive that can readily bond a wide variety of substrates, especially low surface energy materials, such as polyethylene, polypropylene and polytetrafluoroethylene, without requiring complicated surface preparation, priming and the like.
This invention is directed to polymerization initiator systems based on organoborane amine complexes and adhesives and other compositions made therewith. The adhesives are especially useful in bonding low surface energy substrates such as polyethylene, polypropylene and polytetrafluoroethylene.
In 1957 G. S. Kolesnikov et al. (Bull. Acad. Sci. USSR, Div. Chem. Sci. 1957, p. 653) reported the use of tributylborane as a catalyst for the polymerization of styrene and methyl methacrylate. The addition of 2 mole % of tributylborane to methyl methacrylate resulted in rapid polymerization; a transparent solid block was formed in 60 to 90 minutes. At about the same time, J. Furakawa et al. (Journal of Polymer Science, volume 26, issue 113, p. 234, 1957) reported that triethylborane had been found to initiate the polymerization of some vinyl compounds such as vinyl acetate, vinyl chloride, vinylidene chloride, methacrylic ester, acrylic ester, and acrylonitrile. J. Furakawa et al. (Journal of Polymer Science, volume 28, issue 116, 1958) later reported that triethyl borane-catalyzed vinyl polymerization could be markedly accelerated with oxygen or oxygen compounds such as hydrogen peroxide and metal oxides. While the presence of oxygen is apparently needed for the polymerization to occur, the organoborane compounds of the type described in these references are known to be quite pyrophoric in air. Hence, the presence of oxygen is simultaneously required and undesirable.
U.S. Pat. No. 3,275,611 "Process for Polymerizing Unsaturated Monomers with a Catalyst Comprising an Organoboron Compound, a Peroxygen Compound and an Amine" issued Sep. 27, 1966 to E. H. Mottus et al. discloses a process for polymerizing olefinic compounds, especially alpha-olefinically unsaturated compounds. Particularly preferred are methacrylate monomers having no more than 20 carbon atoms in the ester group. The organoboron compound and the amine may be added to the reaction mixture separately or they may be added as a preformed complex. The latter approach reportedly has the advantage of making the boron compound more easily handled, especially for certain boron compounds that tend to be pyrophoric in air but which are not pyrophoric when complexed. Especially useful boron catalysts are said to have the following general formulas: R.sub.3 B, RB(OR).sub.2, R.sub.2 B(OR), R.sub.2 BOBR.sub.2, R.sub.2 BX, and R.sub.2 BH, where R is a hydrocarbon radical, preferably an alkyl radical having from 1 to 10 or more carbon atoms (more preferably, up to 6 carbon atoms), and X is a halogen.
Useful amine complexing agents are said to have a basicity that is preferably in the range of about 10.sup.-6 or 10.sup.-7 to 5.times.10.sup.-10 or 10.sup.-10. Various amine complexing agents are mentioned although pyridine, aniline, toluidine, dimethylbenzylamine, and nicotine are used in the examples. The amine and boron compounds are used in about a 1:1 molar ratio, assuming one nitrogen function per boron function. Reportedly, any peroxide or hydroperoxide compound may be used as a catalyst component.
While Mottus et al. refer to polymerizing methacrylate monomers, there is no indication that the resulting polymers are useful as adhesives. Various acids are mentioned as monomers that may be polymerized but there is no indication that an acid is a component of the polymerization system.
British Patent Specification No. 1,113,722 "Aerobically Polymerisable Compositions," published May 15, 1968 discloses the polymerization of acrylate monomers through the use of a free-radical catalyst (e.g., peroxides) and triarylborane complexes having the general formula (R.sub.3)B-Am wherein R is an aryl radical having from 6 to 12 carbon atoms and Am is, among other things, an amine such as hexamethylenediamine or ethanolamine. The polymerization is activated by heating or the addition of an acid. The resulting compositions are reportedly useful as adhesives.
Chemical Abstracts No. 88532r (volume 73, 1970) "Dental Self-curing Resin" and the full text paper to which it refers report that tributylborane can be made stable in air by complexing it with ammonia or certain amines (e.g., aniline, n-butylamine, piperidine, ethylenediamine) at a mole ratio of one and that the tributylborane can be reactivated with an amine acceptor such as an isocyanate, an acid chloride, a sulfonyl chloride, or anhydrous acetic acid. As a result, the complex can be used to polymerize blends of methyl methacrylate and poly(methylmethacrylate) to provide a dental adhesive. Tributylborane-ethylenediamine complexes and triethylborane-ammonia complexes, each with p-toluenesulfonyl chloride as the amine acceptor, are specifically mentioned.
Chemical Abstracts No. 134385q (volume 80, 1974) "Bonding Polyolefin or Vinyl Polymers" reports that a mixture of 10 parts methyl methacrylate, 0.2 parts tributylborane, and 10 parts poly(methylmethacrylate) was used to bond polyethylene, polypropylene and. poly(vinyl acetate) rods.
U.S. Pat. No. 5,106,928 "Acrylic Adhesive Composition and Organoboron Initiator System," issued Apr. 21, 1992 to M. M. Skoultchi et al., discloses a two-part initiator system that is reportedly useful in acrylic adhesive compositions, especially elastomeric acrylic adhesives. The first part of the two-part initiating system is a stabilizing organoborane amine complex; the second part is an organic acid activator. The organoborane compound of the complex has the general formula: ##STR2## where R, R.sub.1 and R.sub.2 are either alkyl groups having 1 to 10 carbon atoms or phenyl groups, although alkyl groups of 1 to 4 carbon atoms are preferred. The amine portion of the complex may be ammonia, a primary amine, a secondary amine, or a polyamine containing a primary amine or a secondary amine. Useful amines include n-octylamine, 1,6-diaminohexane, diethylamine, dibutylamine, diethylenetriamine, dipropylenediamine, 1,3-propylenediamine, and 1,2-propylenediamine.
The organic acid activator is a compound that will destabilize or liberate the free organoborane compound by removing the amine group, thereby allowing it to initiate the polymerization process. Preferably, the organic acid has the formula R--COOH where R is hydrogen, an alkyl or alkenyl group having 1 to 8 (preferably 1 to 4) carbon atoms, or an aryl group with 6 to 10 (preferably 6 to 8) carbon atoms.
Twelve organoborane amine initiator complexes are described in conjunction with Example 1. In those complexes based on diamines or triamines, the nitrogen atom to boron atom ratio ranges from 2:1 to 4:1. In those complexes based on diethylamine and n-octylamine, the nitrogen atom to boron atom ratio is 1.5:1.
The adhesive compositions are reportedly particularly useful in structural and semi-structural applications such as speaker magnets, metal-metal bonding, (automotive) glass-metal bonding, glass-glass bonding, circuit board component bonding, selected plastic to metal, glass, wood, etc. and electric motor magnets. Those plastics that may be bonded are not further described.